One U.S. official said the government has put off filing charges until at least Monday, The Wall Street Journal reported Sunday. Earlier, the newspaper reported that officials said they might file charges as early as Sunday afternoon.

Tsarnaev, 19, remained at Beth Israel Deaconess Medical Center, and his condition -- with him drifting in and out of consciousness -- was still preventing him from talking to FBI agents, the newspaper said.

It also wasn't clear when the suspect would be able to answer questions. "We don't know if we will ever be able to question the individual," Boston Mayor Tom Menino said in an interview with ABC News .

Officials believe Dzhokhar and his brother Tamerlan Tsarnaev carried out twin bombings near the finish line of the Boston Marathon on Monday that killed three people and injured more than 170. The brothers are also believed to be responsible for killing a campus police officer at the Massachusetts Institute of Technology on Thursday night.

Tamerlan, 26, died following a shootout with police in the Boston suburb of Watertown. Dzhokar was found hiding in a boat Friday evening, weakened by gunshot wounds.

Defense Secretary Chuck Hagel said investigators still had not determined the motivation for the bombings and were probing whether terrorist groups were involved in the attack, The New York Times reported.

But multiple media reports over the weekend said that the FBI had interviewed Tamerlan in early 2011 at the request of the Russian government.

Moscow had expressed concern that the young man planned to travel to Russia and engage in terrorist activities, the Journal reported.

The FBI said it found nothing suspicious, the Journal reported, adding that travel records show Tamerlan traveled to Russia in January 2012 and returned to the U.S. six months later.

The brothers were immigrants of Chechen heritage. Chechnya is a republic of Russia located in the North Caucusus mountains where Islam is the predominant religion. During two separatist struggles against Russian rule in the 1990s, Islamic militants entered the republic.

Multiple media reports said people who knew Tamerlan said he had become more serious about his Muslim faith in the last few years.

In Washington, some lawmakers said Sunday that the surviving suspect should be tried in federal court as a civilian, which would allow prosecutors to seek the dealth penalty, The New York Times reported Sunday.